If you are having trouble getting to a chapter with one credit to unlock it in practice mode, pause game play when you are about to lose your last life. Then, press Start on controller two and use that controller to get to the chapter you wish to unlock. You must not use any continues after that.
khaascp.

Successfully complete the game under the normal difficulty setting without using any continues to unlock the "Game Mode" option at the "Appendix" menu. Alternately, accumulate at least twenty hours of total game play.
wruesch.

Note: This trick requires two Wavebird controllers. Set the receivers on the same channel and use one Wavebird on the same channel. You can then control two players at the same time. Set them side to side (horizontal) or (vertical) above each other by pushing them to the edges of your screen. If one gets wasted, you can quickly change color so you will have one black and the other white.
BlueWhistler3.

Every time you continue, the last digit of your score goes up by one permanently. For example, if you continued once it is a "1", twice for a "2", etc. This is useful for gauging how much you have continued once the "Free Play" option has been unlocked.
bluberry.

Successfully complete the game under the easy difficulty setting without using any continues to unlock the "Sound Test" option at the "Appendix" menu. Alternately, accumulate at least fifteen hours of total game play.
wruesch.

Successfully complete the game under the easy difficulty setting without using any continues to unlock the "Sound Test" option at the "Appendix" menu. Alternately, accumulate at least fifteen hours of total game play.

Successfully complete the game under the normal difficulty setting without using any continues to unlock the "Game Mode" option at the "Appendix" menu. Alternately, accumulate at least twenty hours of total game play.

Accumulate at least three or four hours of total game play to get additional credits. By the time you have accumulated six hours, you should have about nine credits from the start, but it is not unlimited.